During a tornado, the safest place to protect yourself is usually underground, but that's not an option for the large majority of people in southern Oklahoma. If you look just at new construction, fewer than 1 percent of homes in the area hit by the tornado have basements. Here to help explain why is NPR's Scott Neuman, who's written about this for our Two-Way blog.

And Scott, where I come from, a basement is a really common thing to have under the house. Not so in Oklahoma. Why not?

SCOTT NEUMAN, BYLINE: Well, it's interesting. I spoke to a geologist about the soil. There's this red clay that actually absorbs a lot of moisture, and the water table there is very high. And it also has a tendency to dry out in heat, and that causes this contraction and expansion that puts a lot of pressure on concrete- reinforced walls in basements and causes them to crack.

And if they're not properly waterproofed, they will leak. What's interesting is that this geologist actually moved from Texas, where he had been - experienced the 1970 Lubbock, Texas, tornado - which was a pretty big deal there. And so when he moved to Oklahoma, he wanted a house with a basement, and he couldn't find one.

BLOCK: What else did you hear from people, Scott, about why people in this part of Oklahoma wouldn't have a basement?

NEUMAN: Well, the frost line - and that's the level that it freezes down to, in the winter - is actually, fairly high in this part of Oklahoma because it's warm. And the contractors are required by building codes to sink the foundation down below the frost line. So in a place like - say, Indiana, when you excavate to go down below the frost line to put a slab in, you're already halfway there toward a basement whereas in Oklahoma, you don't have to dig down that far. So the upfront costs for a homeowner to put in a basement - is actually a little bit higher.

BLOCK: And what did you hear about the cost of putting in a basement on its own?

NEUMAN: Well, if you talk to a basement contractor in Oklahoma, they'll say that this problem with the clay soil and the moisture and the water table, has kind of been solved. It's really kind of a psychological hangover for people that are used to seeing houses from the '40s and '50s, when the technology wasn't quite as good for waterproofing. And they're saying actually, the cost isn't really that much more to get a good, solid, dry basement.

But people just have this stereotype that basements leak in the area. And as a matter of fact, it actually can be a detriment to resale, for a house to have a basement, because there's this perception that they always leak.

BLOCK: And apart from the basement option, Scott, we've been hearing a lot of talk this week about storm shelters for homes, safe rooms; and there's quite a variety there of options and also costs, right?

NEUMAN: Right. It really depends. I mean, it ranges from a few thousand dollars for a simple shelter that's dug under a garage to maybe 10,000 or more for a more involved shelter below ground outside. And because these homes don't have basements, that's a fairly popular option, to put in a storm shelter. And actually, it's something that local authorities are thinking about.

Right now, there aren't any tornado-specific building codes, but authorities are talking, as we heard earlier in Kirk Siegler's piece, about possibly requiring storm shelters in new buildings.

BLOCK: And in fact, we heard the mayor of Moore, Okla., say he didn't have one before; he's going to get one now.